Commenter Joe Schmoe draws an interesting distinction of relevance to the evacuation debate:

In Katrina, the problem was flooding not the hurricane storm itself.

In Harvey, the problem was flooding not the hurricane storm itself.

In Katrina, people were told to evacuate, and competent people did.

In Harvey, people were told not to evacuate, and the competent people didn’t.

In Katrina, there were needy but very few competent helpers.

In Harvey, there were needy and all the competent people were still in the area to act as helpers.